Given a list of unique words, return all the pairs of the distinct indices (i, j) in the given list, so that the concatenation of the two words words[i] + words[j] is a palindrome. Example 1: Input: words = ["abcd","dcba","lls","s","sssll"] Output: [[0,1],[1,0],[3,2],[2,4]] Explanation: The palindromes are ["dcbaabcd","abcddcba","slls","llssssll"] Example 2: Input: words = ["bat","tab","cat"] Output: [[0,1],[1,0]] Explanation: The palindromes are ["battab","tabbat"] Example 3: Input: words = ["a",""] Output: [[0,1],[1,0]] Constraints: 1 <= words.length <= 5000 0 <= words[i].length <= 300 words[i] consists of lower-case English letters.